McMillan (27) will compete in the 4x200m freestyle relay with Matt Stanley (Matamata), Steve Kent (Auckland) and Dylan Dunlop-Barrett (Taranaki).
They broke the New Zealand record by seven seconds at the national championships at Auckland and qualified for the Olympics, because the time of 7min 14sec ranked them 10th in the world.
"It's been my dream for my whole life," McMillan said.
"I've been swimming since I was 8 and every swimmer wants to swim on the biggest stage in the world, and that's the Olympics."
Swimmers have to do qualifying times during a national championships but McMillan caught a virus when returning from his training home on the Gold Coast and failed to qualify in the 200m freestyle and the 200m butterfly, his best individual events.
"I managed to find something in the relay and went two seconds quicker that I swam in the individual race," he said.
"The day after that I crashed and had to pull out of the rest of the meet."
Scott Cameron, of Otago, qualified in the 4x200m freestyle relay for the 1996 Olympic Games in Atlanta and was also able to compete in the 1500m.
Fina has tightened its qualifying standards since then and that option was not available for McMillan.
He has the chance of a swim-off for the 100m butterfly position in the medley relay against Daniel Bell on July 27 in London. They have both reached the Fina B standard.
"We've got a good relay team and enjoy each other's company," McMillan said.
McMillan had been tipped as an Olympian since his time training with Heather Third at the Cromwell Pool from 1995-99.
He came to Dunedin and trained with Duncan Laing for the next few years and was expected to qualify for the 2004 Olympics at Athens. But a bout of glandular fever halted his progress.
McMillan shifted to Auckland and trained at the Millennium Institute with Jan Cameron in a bid to make the Beijing Olympics in 2008 but missed out again.
"I'd be lying if I said those thoughts didn't come into my head," he said.
"Over the years, I've had such a bad run with injuries."
In 2006 he broke his ankle while pig hunting near Ranfurly and broke the same ankle four years later, in 2010. He also broke his tailbone two years ago when he had an accident playing touch rugby with fellow swimmers.
Serious questions came into his head as he pondered his fate.
"Do I have enough time to make it to the Olympics?
"Can I be fit enough?" he said.
"Little things like that held me back.
"But I never lost faith in my own ability. I always knew I could do it but I had to dig deep."
Duncan Laing told McMillan that he could compete at the Olympics.
"I had to remind myself of things like that," he said.
"I was never going to give up and give less than 100%. I have finally got there."
His shift to the Queensland Gold Coast to train at the Southport Club with coach Glen Baker made the difference.
"It was the best place for me to be in," he said. "I love surfing and Gold Coast living is pretty relaxed.
"I surrounded myself with good support people and other athletes who had same same goals."
Five swimmers from Baker's squad have made the Australian Olympic team.
"We are calling ourselves the Southport six," he said.
